What is an allergy?

Allergy or allergic reactions are exaggerated inappropriate responses to a harmless molecule. Allergens are molecules that are chemically inactive. There are two types of allergic responses:

  1. Early phase response: this is the immediate response when exposed to allergen and symptoms are florid. This phase lasts minutes to hours and is mediated by chemical substances in the body called histamine and serotonin which lead to bronchoconstriction and other symptoms that a patient may present with. Individuals in this phase have a good response to antihistamines.
  2. Late phase response: This phase occurs few hours after exposure to an allergen and lasts several days. In this phase, white blood cells (Eosinophils) are released and persist for 2 to 3 days thereby encouraging the release of chemical mediators (leukotrienes) which cause extensive damage to the body tissues. This phase does not respond to antihistamines but responds to immunotherapies and steroids.


Atopy is the tendency to develop allergic response to inert (chemically inactive) molecules and this is inherited (runs in families). An atopic parent will pass it on to 50% of his/her children.  Allergens are mostly able to cross skin and mucous membrane barriers (bronchioles, nasal mucousa, conjunctiva). Individuals with atopy have more active Th2 pathway cells which recognize allergens and mount a response which then leads to the symptoms. Scientists have postulated that reduced exposure to germs early in life favours the Th1 pathway and therefore when exposed later in life to germs, Th2 pathway is activated and the reaction is more sustained. They have circulatory allergen-specific IgE.

Bronchial asthma is one of the most common allergic reactions, others include

  • Allergic rhinitis
  • Allergic eczema
  • Bee and wasp venom allergies
  • Cat allergy
  • Water allergy
  • Food allergy

Allergic rhinitis

Allergic rhinitis is characterized by nasal mucousal itching, engorgement, discharge, and congestion

Allergy to cats

Cat allergy is one of the most common allergies in humans. People who are allergic to cats often react to the allergens in the sebaceous glands (glands on cat skin). They have produced specific antibodies against the cat allergen which subsequently aids to mount an immune response whenever they are exposed to same allergen. These allergens are also present in the saliva, fur (dander) and anus of the cats.

The inhaled allergen activates mast cells with increased mucous production and airway constriction , leading to cough, wheezing ,and difficulty breathing.

Symptoms of allergy to cat

The symptoms of allergic reaction to cats range from mild to severe. Mild to moderate symptoms include

  • cough,
  • wheeze
  • sneeze
  • itchy and stuffy nose
  • swollen watery eyes
  • itchy throat
  • ear pain
  • sore throat
  • and in more severe cases rhinitis and bronchial asthma.
allergy symptoms
Symptoms of allergic reactions Credit: Timonina/Shutterstock

Cat allergy reactions may be prevented by getting cat species that are known to be hypo allergic, female cats are preferable for individuals have cat allergies because females produce less allergens.. Frequently giving baths to the cat at least once in two days may help to reduce the amount of allergen on the furs of the cat thereby reducing exposure to allergen. Avoiding cats altogether is the best way to prevent cat allergy.

In the treatment of cat allergy, antihistamines and nasal decongestants may be used, steroids are used in severe cases.

Allergy to water

Water allergy is also known as aquagenic urticaria, an extremely rare disease in which skin contact with water(whether hot or cold) triggers allergic reaction in form of skin rashes such as itchy wheals and urticaria. In severe cases they may present with angioedema which is swelling under skin and mucous membranes. The rashes are more often in the neck, torso and upper limbs. Once the skin is dry, for most people, the wheals will clear in 30 to 60 minutes. The individuals with this condition usually do not react to drinking water but to having water touch their skin. It resolves spontaneously for most  individuals after a few years.

Treatment is with antihistamines, topical petrolatum (acts as a barrier between skin and water), phototherapy and biologics.

Food allergy

Some people react to certain foods, most commonly milk, eggs, nuts, and sea foods. Milk allergy usually starts in childhood due to an abnormal response to protein in milk. Milk allergy is most commonly seen with cow milk but also seen with goat milk and other types of animal milk. Egg allergies may be to both the albumen and yolk of the egg although allergy to egg albumen is said to be more common.

Those at risk of developing food allergies include children, people with other allergies, people that have atopic dermatitis, and those with family history of atopy.

Most food allergies start in childhood and may resolve spontaneously after a few years.

How is allergy diagnosed?

Diagnosis of allergies may be made by appropriate history taking and confirmed with skin prick testing. Generally, allergies may be managed by avoiding the allergen, use of antihistamines, corticosteroids, leukotriene receptor antagonists and allergen immunotherapy or desensitization.

Allergy and asthma

To read more about allergy as an asthma trigger.

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